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* Re: [TUHS] Unix for PDP11/20 w/o mmu or paging option
@ 2021-09-17 20:18 Noel Chiappa
  2021-09-17 20:47 ` Clem Cole
  0 siblings, 1 reply; 17+ messages in thread
From: Noel Chiappa @ 2021-09-17 20:18 UTC (permalink / raw)
  To: tuhs; +Cc: jnc

    > From: Clem Cole

    > The KS11 MMU for the 11/20 was built by CSS ... I think Noel has done
    > more investigation than I have.

I got a very rough description of how it worked, but that was about it.

    > I'm not sure if the KS11 code is still there. I did not think so.

No, the KS11 was long gone by later Vn. Also,I think not all of the -11/20
UNIX machines had it, just some.

    > The V1 work was for a PDP-7

Actually, there is a PDP-11 version prior to V2, canonically called V1.
The PDP-7 version seems to be called 'PDP-7 UNIX' or 'V0'.

    > I'm fairly sure that the RK05,  used the RK11-D controller.

Normally, yes. I have the impression that one could finagle RK03's to work on
the RK11-D, and vice versa for RK05's on the RK11-C, but I don't recall the
details. The main difference between the RK11-C and -D (other then the
implementation) was that i) the RK11-C used one line per drive for drive
selection (the -D used binary encoding on 3 lines), and ii) it had the
'maintenance' capability and register (al omitted from the -D).

    > The difference seems to have been in drive performance.

Yes, but it wasn't major. They both did 1500RPM, so since they used
the same pack format, the rotational delay, transfer rate, etc were
identical. The one peformance difference was in seeks; the
average on the RK01 was quoted as 70 msec, and 50 msec on the

    > Love to see the {KT11-B prints] and if you know where you found them.

They were sold on eBait along with an -11/20 that allegedly had a KT11-B. (It
didn't; it was an RK11-C.) I managed to get them scanned, and they and the
minimal manual are now in Bitsavers. I started working on a Tech Manual for
it, but gave up with it about half-way done.

    > I wonder if [our V1 source] had the KS-11 stuff in it. 

No; I had that idea a while back, looked carefully, our V1 listings
pre-date the KS11.

    > From: Roland Huisman

    > There is a KT11B paging option that makes the PDP11/20 a 18 bit
    > machine.

Well, it allows 2^18 bytes of main memory, but the address space of the
CPU is still2^16 bytes.

    > It looks a bit like the TC11 DECtape controller.

IITC, it's two backplanes high, the TC11 is one. So more like the RK11-C...

    > I have no idea how it compares to the later MMU units from the
    > software perspective.

Totally different; it's real paging (with page tables stored in masin
memory). The KT11-B provides up to 128 pages of 512 bytes each, in both Exec
and User mode. The KT11-C, -D etc are all segmentation, with all the info
stored in registers in the unit.

    > I wonder is there is some compatibility with the KT11-B [from the KS11]

I got the impression that the KS11 was more a 'base and bounds' kind
of thing.


^ permalink raw reply	[flat|nested] 17+ messages in thread
* Re: [TUHS] Unix for PDP11/20 w/o mmu or paging option
@ 2021-09-17 15:35 Noel Chiappa
  2021-09-17 18:28 ` Phil Budne
  0 siblings, 1 reply; 17+ messages in thread
From: Noel Chiappa @ 2021-09-17 15:35 UTC (permalink / raw)
  To: tuhs; +Cc: jnc

    > From: Roland Huisman

    > I have a PDP11/20 and I would love to run an early Unix version on
    > it. ... But it seems that the earliest versions of Unix do not need the
    > extra memory.  Does anyone have RK05 disk images for these early Unix
    > versions?

Although the _kernel_ source for V1 is available:

most of the rest is missing; only 'init' and 'sh' are available. So one would
have to write almost _everything_ else. Some commands are available in PDP-11
assembler in later versions, and might be movable without _too_ much work -
but one would have to start with the assembler itself, which is luckily in

If I were trying to run 'UNIX' on an -11/20, I think the only reasonable
choice would be MINI-UNIX:

It's basically V6 UNIX with all use of the PDP-11 memory management
removed. The advantage of going MINI-UNIX is that almost all V6 source
(applications, drivers, etc) will run on it 'as is'.

It does need ~56KB of main memory. If you don't have that much on the -11/20,
LSX (links in the above) would be an option; it's very similar to MINI-UNIX,
but is trimmed down some, to allow its use on systems with less main memory.

I'm not sure if MINI-UNIX has been run on the -11/20, but it _should_ run
there; it runs on the -11/05, and the only differences between the /20 and the
/05 are that the /20 does not have the RTT instruction (and I just checked,
and MINI-UNIX doesn't use RTT), and SWAB doesn't clear the V condition code
bit. (There are other minor differences, such as OP Rn, (Rn)+ are different on
the -11/20, but that shouldn't be an issue.)

Step 1 would be to get MINI-UNIX running on an -11/20 under a simulator; links
in the above to get you there.


^ permalink raw reply	[flat|nested] 17+ messages in thread
[parent not found: <>]

end of thread, other threads:[~2021-09-17 21:22 UTC | newest]

Thread overview: 17+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-09-17 20:18 [TUHS] Unix for PDP11/20 w/o mmu or paging option Noel Chiappa
2021-09-17 20:47 ` Clem Cole
  -- strict thread matches above, loose matches on Subject: below --
2021-09-17 15:35 Noel Chiappa
2021-09-17 18:28 ` Phil Budne
2021-09-17 19:43   ` Clem Cole
2021-09-17 19:51     ` Brantley Coile
2021-09-17 20:07       ` Clem Cole
2021-09-17 21:21       ` Phil Budne
     [not found] <>
2021-09-17 14:09 ` Roland via TUHS
2021-09-17 14:33   ` Warner Losh
2021-09-17 15:32     ` Clem Cole
2021-09-17 16:04       ` Roland via TUHS
2021-09-17 16:14         ` Clem Cole
2021-09-17 16:44           ` Roland via TUHS
2021-09-17 16:13       ` Warner Losh
2021-09-17 16:17         ` Clem Cole
2021-09-17 16:05     ` Roland via TUHS

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